Zeolites are microporous crystalline aluminosilicates that have found important uses as catalysts, adsorbents and ionic exchangers. Many of these zeolitic materials have well defined structures that form channels and cavities inside themselves which because of having a uniform size and shape permit adsorption of certain molecules, whereas they prevent other molecules—which are too large to diffuse through the pores—from passing inside the crystal. This property gives these materials molecular sieve properties. These molecular sieves can include in the lattice Si and other elements of group IIIA of the periodic system, all of them tetrahedrally coordinated, the tetrahedrons being attached by their vertexes by means of oxygen atoms of the lattice. The negative charge produced by the elements of group IIIA tetrahedrally coordinated in lattice positions is compensated by the presence in the crystal of cations such as for example alkali or alkaline earth metals. A type of cation may be totally or partially exchanged by another type of cation by ionic exchange techniques. By means of cationic exchange it is possible to vary the properties of a given silicate by selecting the desired cation.
Many zeolites have been synthesized in the presence of an organic molecule that acts as a structure directing agent. Very frequently these organic molecules that act as structure directing agents (SDA) contain nitrogen in their composition that can give rise to stable organic cations in the reaction medium.
Mobilization of silica may be carried out in the presence of OH− groups and a basic medium that may be introduced by the SDA itself, such as for example, tetrapropylammonium hydroxide in the case of zeolite ZSM-5. It is also known that fluoride ions can also mobilize silica for the synthesis of zeolites, and the use of HF in H2O at a low pH as a mobilizing agent of silica for the synthesis of ZSM-5 has been described, for example, in European patent EP-337479.
In the same way zeolite ITQ-17 has been synthesized in a conventional manner in the synthesis of zeolites, as described in patent application PCT/ES01/00385, by means of a process that comprises the use of fluorides.
However, the use of fluoride ions in synthesis is less desired from an industrial point of view than the use of OH−, given that the presence of fluoride ions requires the use of special materials in the synthesis equipment, as well as a specific treatment of the waste water and gases.